Of the numerous methods that are available to solid-state chemistry, hydrothermal synthesis represents the most suitable method for synthesizing zeolites. The first zeolite syntheses were carried out in 1862 by St. Claire Deville (levyne) and in 1882 by De Schulten (analcime) under hydrothermal conditions.
The first zeolite syntheses which led to uniform and clearly characterized products took place around the middle of the last century. Thus, using reactive aluminosilicate gels, Milton succeeded in synthesizing zeolites with large pore openings and correspondingly high adsorption capacities. At approximately the same time, Barrer and his colleagues established the principles of modern zeolite synthesis by carrying out systematic studies and improving methods.
The educts which are necessary for a zeolite synthesis can be divided into the following five types:                source of the T-atoms, thus e.g. a silicon and/or an aluminium source,        templates,        mineralizers,        solvents, and        possibly seed crystals.        
The T-atoms are limited by the following selection rules:                ion radius ratio R(Tn+)/R(O2−) of from 0.223 to 0.414 (Pauling's rule),        the electronegativity allows a stable bond with oxygen,        the oxidation state lies between +2 and +5,        the solubility improves due to polyanions or formation of TO2− units,        the resultant framework has charge centres with a charge between −1 and 0.        
As a rule, the following silicon sources are used for the synthesis of zeolites:                sodium water glass (sodium silicate solutions), alkali metasilicates and alkali water glass solutions,        precipitated silicas,        pyrogenic silicic acids, SiO2-rich flue ash and insoluble silicates,        silicon-rich aluminas,        silica sols (colloidally dissolved SiO2),        organosilicon compounds (usually orthosilicic acid esters).        
The following are often used as aluminium sources:                aluminium salts of mineral acids or organic acids,        aluminium oxides or hydroxides and sols thereof (colloidal solutions)        aluminium alcoholates        elemental aluminium        aluminium-rich earths, aluminium-rich flue ash        
As a rule, templates are mono- or polyvalent inorganic or organic cations. The templates have structure-directing properties and stabilize the zeolite structure that forms during the synthesis. Some representative examples may be named:                Sr2+ or K+ for the synthesis of the zeolite ferrierite (FER)        tetramethylammonium cations for the synthesis of ZSM-5 (MFI),        K+ [18-crown-6] for the synthesis of the zeolites MCM-61 (MSO) or EMC-2 (EMT),        [bis(pentamethylcyclopentadienyl) Co(II)]+ complex for the synthesis of zeolite UTD-1 (DON).        
However, in some syntheses uncharged organic compounds are also used, such as e.g. pyrrolidine or ethylenediamine in the synthesis of ZSM-35 (FER).
Hydroxide ions mainly serve as mineralizers. One of the most important functions of the mineralizers is to dissolve the amorphous aluminosilicates during the synthesis. However, mineralizers also serve to increase the solubility of the species that contain the T-atoms. This is brought about e.g. by achieving an ideal pH (possibly additional buffer action), increasing the ionic strength (addition of foreign ions) or complexing the T-atom. Therefore, other soluble compounds which have e.g. complex-forming properties are also often used as mineralizers.
Water is mainly used as solvent. However, there are also syntheses in non-aqueous solvents, such as e.g. in pyridine or glycerol. The admixing of alcohols to aqueous synthesis gels is likewise possible.
The formation mechanism concepts for zeolite synthesis can be described as follows. In the majority of cases, synthesis gels are prepared in a sol-gel process. Ideally, the starting point is a solution, or several solutions, colloids (sols) or suspensions which, by suitable blending, are converted to an amorphous gel which crystallizes out under hydrothermal conditions. For a clearer understanding, the zeolite synthesis can be divided into the following part-steps:                gel preparation,        achieving supersaturation,        nucleation, and        crystal growth.        
The molar composition of the synthesis gel is the most important factor that determines the reaction products. The following notation is usually used:aSiO2:Al2O3:bMxO:cNyO:dR:eH2OM and N stand for e.g. alkali or alkaline-earth ions and R for an organic template. Furthermore, the coefficients a to e indicate the molar ratios relative to one mole aluminium(III)oxide. During the gel preparation, the individual starting mixtures are blended, and if necessary the required pH is set. An amorphous precipitate forms (e.g. an aluminosilicate). The obtained products (e.g. their crystallite size) can be influenced by ageing, thermal treatment or ultrasonic treatment of the starting mixtures or of the synthesis gels.
After the formation of the precipitate, it is important to achieve supersaturation as this has an influence on the nucleation. Above all, the mineralizers are necessary for the supersaturation as they partially dissolve the high-molecular-weight system either already during the gel preparation (in an ageing step) or during the synthesis (before the actual nucleation) and provide smaller agglomerates. These can be dissolved in the aqueous phase again, possibly form further crystal nuclei, enlarge existing crystal nuclei or be incorporated into existing crystallites (R. A. Rakoczy, Hydrothermalsynthese ausgewählter Zeolithe und ihre Charakterisierung durch Adsorption, Dissertation, University of Stuttgart, 2004).
The zeolite L that is relevant within the framework of this invention (also called aluminium silicate LTL) has a zeolite system with 12-ring pores which crystallizes with a hexagonal unit cell. Zeolite L has a one-dimensional pore system with a pore diameter of approximately 7.1 Å and an average silicon-aluminium ratio of approximately 6. Because of this large pore diameter, zeolites L are particularly suitable for catalytic reactions of large molecules such as long-chain aliphatic compounds (C. Baerlocher, W. M. Meier, D. H. Olson, Atlas of Zeolite Framework Types, fifth revised edition 2001, Elsevier, Amsterdam-London-New York-Oxford-Paris-Shannon-Tokyo, page 170). LTL-type aluminium silicates display a typical X-ray diffraction pattern with the following dominant reflections (Cu Kα1 with a wavelength of 1.5406 Å) and typical d(Å) values:
2 Theta (°)d (Å)5.5415.911.757.514.696.015.205.819.274.620.454.322.643.924.303.625.573.527.093.327.973.229.073.129.643.030.682.933.722.734.182.635.822.536.962.441.002.248.371.9
The first explanations of the synthesis of the zeolite L can be found in U.S. Pat. No. 3,216,789. In a synthesis typical for this, a hydrated alumina is converted to a potassium aluminate by being dissolved in potassium hydroxide. Colloidal silica sol is then admixed. A synthesis gel with the following composition is obtained:8K2O:Al2O3:20SiO2:200H2O
The synthesis gel is kept at a temperature of 100° C. for 196 hours in a closable glass vessel. This crystallization process takes place without stirring. The zeolite L is obtained after separation and washing. This material is characterized by a characteristic X-ray powder diffractogram.
The synthesis of zeolite L is not usually simple, as often no phase-pure zeolite L is obtained, but zeolite W (MER) is additionally formed. Zeolite W is a small-pore zeolite (8-ring pores, diameter of less than 5 Å) which grows simultaneously in the synthesis gel that is intended for the production of zeolite L. Zeolite W displays an X-ray diffraction pattern with the following dominant reflections (Cu Kα1 with a wavelength of 1.5406 Å) and typical d(Å) values:
2 Theta (°)d (Å)12.47.112.57.016.55.316.65.317.75.017.84.927.33.227.63.228.03.128.23.130.32.930.42.9
The presence of a by-product, such as e.g. zeolite W, can drastically reduce the activity of the large-pore zeolite L. The production of a high-purity zeolite L is therefore desirable.
U.S. Pat. No. 5,242,675 discloses a synthesis method which is intended to lead to phase-pure zeolite L. Aluminium hydroxide is dissolved in an alkaline lye by heating. After supplementing the evaporated quantity of water, a dilute colloidal silica sol is added and a synthesis gel is obtained, accompanied by stirring, which is crystallized out at 150° C. within 72 hours. A stainless steel pressure vessel lined with Teflon is used.
The crystallization batch was not stirred during the synthesis. A characteristic of the process is that the formation of zeolite W as a foreign phase is suppressed during the preparation of the synthesis gels by adding metal salts (e.g. barium, magnesium or calcium hydroxide) to the dilute colloidal silica sol solution.
A further production process for zeolite L is described in U.S. Pat. No. 4,530,824. In a first step, an “amorphous compound” which consists of a sodium aluminosilicate is produced, washed, added to a potassium hydroxide solution and crystallized out under hydrothermal conditions.
The silicon sources given above differ greatly in price. Thus an aqueous solution of sodium silicate, in particular a commercial sodium water glass solution, is much less expensive than a precipitated silica, a pyrogenic silicic acid or even a colloidal silica sol. This is similarly true for possible aluminium sources, thus aluminium sulphate is much less expensive than a sodium aluminate or an activated aluminium hydroxide.
However, the use of less expensive sodium silicate solutions is often ruled out, as a direct preparation of a synthesis gel from a sodium silicate solution leads to a composition which has too much sodium. In these cases, more expensive sources, such as precipitated silicas or colloidal silica sols, must therefore be used.
In the commercial production of zeolites and zeolite-like materials, the phase purity of the obtained product and the cost-effectiveness of the formulations used, i.e. the costs of the raw materials used and the yields obtained, in particular are of most interest.
There is therefore a need for an improved process for producing zeolite L and preliminary stages therefor. Consequently, the object of the invention is to provide a production process for zeolite L and preliminary stages therefor which can be carried out using more reasonably priced educts and which can be used in particular to produce a phase-pure zeolite L. The object is achieved by the preparation of an amorphous aluminium silicate precursor (mixed hydroxide) which can be modified subsequently and can be produced by precipitation using inexpensive silicon and aluminium sources, and from which a zeolite L can be obtained by further process steps via a synthesis gel.